This invention relates to the monitoring and control of acid in the overhead water of oil refineries. Specifically, this invention constitutes an improved method for controlling these acids, by continually monitoring their concentration and by controlling the rate of introduction of neutralizing agent in response to this continually monitored concentration.
Corrosive aqueous acids are present in distillation towers used in petroleum refining. Water condenses at the "dew point" near the top of these distillation towers. Dissolved in this water are hydrochloric acid, organic acids, and various other acidic species, which rapidly corrode exposed areas in the vicinity of the dew point and points downstream.
Various methods have been suggested to remedy this corrosion. Generally, a neutralizing agent is introduced, either directly to the crude charge or injected at the top of the distillation tower. The quantity of neutralizing agent to be added is usually determined following some manual sampling and acid analysis of the tower condensate or of some other species. Because the time elapse between these manual analyses may span one or more days, excursions in the acid concentrations of the overhead water may occur. Because of the rapidity of acid corrosion, even a one day's unmonitored and uncontrolled excursion can have serious consequences.
Thus there is a need for a method of continually monitoring the acid concentration of overhead water, and of controlling the rate of introduction of neutralizing agent in a continuous manner, to avoid acid corrosion. One such system is described by Richmond and Browne in "Automatic pH Control of Crude Column Overhead Streams", presented at the International Corrosion Forum of March 1982. This system consists broadly of a sampling system, a pH cell, a neutralizer system, and a control system responsive to the measured pH. However, this system is vulnerable to cell fouling by hydrocarbon contamination during plant upsets, necessitating manual control during such upset, pending cleaning of the electrodes. Furthermore, a measurement of pH may underestimate the quantity of neutralizer required, due to the buffering effect of weak acids in the system. These and other problems are addressed in the present invention.